The present invention relates to a grid-inserted quantum structure which is used as an element for controlling electrons or light.
Elements which employ quantum structures such as quantum well lasers, quantum box lasers, quantum wire lasers (transistors) and the like have been proposed as devices employing new ideas which utilize the techniques of growing structures such as the molecular beam epitaxy method and the organometallic CVD method.
A quantum well laser is designed with an active layer having a quantum structure with a thickness (of about 100 .ANG.) substantially the same as the de Broglie wavelength .lambda. of electrons so that electrons can be captured in a quantum manner in the direction of the thickness and can behave as free particles only in the two-dimensional directions along the thickness of the layer. Such a quantum well laser is characterized in that the oscillation wavelength can be controlled by controlling the structure, e.g., the thickness of the layer, and excellent oscillation characteristics can be obtained at a threshold current value.
In the above-described quantum well laser in which electrons are captured in a quantum manner in the direction (z) of the thickness of the structure, electrons are further captured in a quantum manner in the two-dimensional directions (x, y) along the thickness to form a quantum box layer. Electrons are captured in a quantum manner in either of the directions to form a quantum wire laser.
In order to realize a device employing the above-described structure, it is necessary to reduce the size of the quantum wire and box. Although it is preferable that the crystal structure has a size of several hundred .ANG. or less, generally 500 .ANG. or less, it is difficult to form such a device by using conventional electron beams or fine processing.
FIG. 1 is a drawing of an example of a two-dimensional step structure; FIG. 2, a drawing for explaining a method of forming a periodic step structure; FIG. 3, a drawing for explaining a method of forming a quantum well device using the method of growing a crystal; and FIG. 4, a drawing of an example of a quantum well device in which crystals having different compositions are formed in the longitudinal direction. In these drawings, reference numeral 21 denotes a substrate; reference numeral 22, a structure; reference numerals 23, 24, barrier layers, and reference numeral 25, a confining layer.
In FIG. 2, the substrate 21 is composed of, for example, a GaAs crystal, which is polished at a specific angle .phi. in a specific direction to obtain a periodic step structure as shown in the drawing. Assuming that a layer comprising atoms each denoted by O is formed, a step structure with steps each having a thickness corresponding to one atomic layer is formed by polishing the layer so as to scrape off atoms each denoted by O shown by a dotted line. That is, as a single atom cannot be partially polished, the atoms which are partially subjected to polishing (denoted by 0 shown by a dotted line) are scraped off to form a step on the atomic unit. Thus, the width of a step depends upon the polishing angle in such a manner that the width decreases as the angle .phi. increases and, contrarily, it increases as the angle decreases. For example, the angle .phi. and the step width .LAMBDA. have the following relation:
______________________________________ .PHI. (.degree.) 0.5 0.8 1 1.6 2 .LAMBDA. ( .ANG. ) 320 200 160 100 80 ______________________________________
It is also possible to form the two-dimensional step structure shown in FIG. 1 by appropriately selecting the directions of polishing.
On the thus-formed substrate 21 are deposited material A and material B, as shown in FIG. 3, to form a crystal having different compositions in the longitudinal direction. In other words, when the material A is first deposited in a thickness corresponding to several atoms, since the material is bonded to the lower and side surfaces at the corner of each step, the crystal successively grows from the corner In this way, a confining layer 25 can be formed between barriers 23, 24 each serving as an electron-capturing layer, as in the structure shown in FIG. 4.
In the above-described crystal, however, it is necessary to stack, for example, the material A on the same material A over a thickness corresponding to 30 to 40 atomic layers. However, there is a problem in that a plurality of atomic layers cannot be easily placed on the same material, and such atomic layers cannot be easily deposited unless the controllablity of deposition is improved. It is also difficult to form a quantum box structure.